Magicicada spp. spend most of their 13- and 17-year lives underground feeding on xylem fluids from the roots of deciduous forest trees in the eastern United States.[3] After 13 or 17 years, mature cicada nymphs emerge at any given locality, synchronously and in tremendous numbers. After such a prolonged developmental phase, the adults are active for about 4 to 6 weeks.[4] The males aggregate into chorus centers and attract mates. Within two months of the original emergence, the life cycle is complete, the eggs have been laid and the adult cicadas are gone for another 13 or 17 years.

The familiar winged imago (adult) periodical cicada has red eyes and a black dorsal thorax. The wings are translucent and have orange veins. The underside of the abdomen may be black, orange, or striped with orange and black, depending on the species.[5]

Adults are typically 2.4 to 3.3 cm (0.9 to 1.3 in), depending on species, slightly smaller than the annual cicada species found in the same regions of the United States. Mature females are slightly larger than males.[6]

Magicicada males typically form large aggregations that sing in chorus to attract receptive females. Different species have different characteristic calling songs. The call of decim periodical cicadas is said to resemble someone calling "weeeee-whoa" or "Pharaoh."[7] The cassini and decula periodic cicadas have songs that intersperse buzzing and ticking sounds.[6]

Cicadas do not bite or sting. Like other Auchenorrhyncha, they have mouthparts used in piercing plants and sucking their sap. A cicada's proboscis can also pierce human skin when it is handled, which is painful but in no other way harmful. These cicadas are not venomous, and there is no evidence that they transmit diseases. They pose little threat to mature vegetation, although planting new trees or shrubs is best postponed until after an expected emergence of the periodical cicadas. Mature plants rarely suffer lasting damage, although twig die-off or flagging can result from egg-laying.[8]

Transformation of the periodical cicada from the mature nymph to the adult

Nearly all cicadas spend years underground as juveniles, before emerging above ground for a short adult stage of several weeks to a few months. The seven periodical cicada species are so named because, in any one location, all of the members of the population are developmentally synchronized—they emerge as adults all at once in the same year. This periodicity is especially remarkable because their life cycles are so long—13 or 17 years. Cicadas of all other species (perhaps 3000 worldwide) are not synchronized, so some adults mature each summer and emerge while the rest of the population continues to develop underground. Many people refer to these non-periodical species as annual cicadas since some are seen every summer. The life cycles of most annual species range from two to ten years, although some could be longer.

The nymphs of the periodical cicadas live underground, often at depths of 30 cm (1 ft) or more, feeding on the juices of plant roots.[9] The nymphs of the periodical cicada undergo 5 instar stages in their development underground. The difference in the 13 and 17 year life cycle is the time it takes for the second instar to mature. While underground, the nymphs move deeper below ground feeding on larger roots.[10]

Emergence holes

Magicicada molting

Magicicada in final molting stage prior to hardening of exoskeleton

The nymphs emerge on a spring evening when the soil temperature at about 20 cm (8 in) depth is above 17 °C (63 °F). In most years, this works out to late April or early May in far southern states, and late May to early June in the far northern states. Emerging nymphs climb to a suitable place on the nearby vegetation to complete their transformation into an adult cicada. They molt one last time and then spend about six days in the leaves waiting for their exoskeleton to harden completely. Just after this final molt, the teneral adults are white, but darken within an hour.

Adult periodical cicadas live only for a few weeks—by mid-July, all have disappeared. Their short adult life has one purpose: reproduction. The males "sing" a species-specific mating song; like other cicadas, they produce loud sounds using their tymbals. Singing males of a single Magicicada species form aggregations (choruses) that are sexually attractive to females. Males in these choruses alternate bouts of singing with short flights from tree to tree in search of receptive females.[11] Most matings occur in "chorus" trees.[5]

Receptive females respond to the calls of conspecific males with timed wing-flicks, which attract the males for mating.[12] The sounds of a "chorus"—a group of males—can be deafening and reach 100 dB. In addition to their "calling" or "congregating" song, males produce a distinctive courtship song when approaching an individual female.[5]

Magicicada egg slits (circled in red)

Both males and females can mate multiple times, although most females seem to mate just once. After mating, the female cuts V-shaped slits in the bark of young twigs and lays approximately 20 eggs in each, for a total of 600 or more eggs. After about six to ten weeks, the eggs hatch and the newborn nymphs drop to the ground, where they burrow and begin another 13 or 17-year cycle.

The nymphs emerge in large numbers at about the same time, sometimes more than 1.5 million individuals per acre (>370/m²).[13] Their mass-emergence is a survival trait called predator satiation: for the first week after emergence, the periodical cicadas are an easy prey for reptiles, birds, squirrels, cats, and other small and large mammals.[4][14] Early ideas maintained that the cicadas' overall survival mechanism was simply to overwhelm predators by their sheer numbers, ensuring the survival of most of the individuals. It was hypothesized that the emergence period of large prime numbers (13 and 17 years) was a predator avoidance strategy adopted to eliminate the possibility of potential predators receiving periodic population boosts by synchronizing their own generations to divisors of the cicada emergence period.[15] Another viewpoint holds that the prime numbered developmental times represent an adaptation to prevent hybridization between broods with different cycles during a period of heavy selection pressure brought on by isolated and lowered populations during Pleistocene glacial stadia, and that predator satiation is a short term maintenance strategy.[16] This hypothesis was subsequently supported through a series of mathematical models, and stands as the most widely accepted explanation of the unusually lengthy and mathematically precise immature period of these insects.[17] The length of the cycle was hypothesized to be controlled by a single gene locus, with the 13-year cycle dominant to the 17-year one.,[18] but this interpretation remains controversial and unexplored at the DNA level.

Cycles in cicada populations are significant enough to affect other animal and plant populations. For example, tree growth has been observed to decline the year before the emergence of a brood, because of the increased feeding on roots by nymphs.[19]Moles, which feed on nymphs, have been observed to do well during the year before an emergence, but suffer population declines the following year, because of the reduced food source.[20]Wild turkey populations respond favorably to increased nutrition in their food supply from gorging on cicada adults on the ground at the end of their life cycle. Uneaten carcasses of periodic cicadas decompose on the ground, providing a resource pulse of nutrients to the forest community.[19]

Cicada broods may also have a negative impact. It has been suggested that Squirrel populations have been negatively impacted, because the egg laying activity of female cicadas damaged upcoming mast crops.[citation needed]

Periodical cicadas are grouped into broods based on the calendar year when they emerge (see chart below and maps on www.magicicada.org). For example, in 2014, 13-year Brood XXII is scheduled to emerge in Louisiana and 17-year Brood III is scheduled to emerge in western Illinois and eastern Iowa.

In 1898, entomologist C. L. Marlatt assigned Roman numerals to 30 different broods of periodical cicadas: 17 distinct broods with a 17-year life cycle, to which he assigned brood numbers I through XVII (with emerging years 1893 through 1909); plus 13 broods with a 13-year cycle, to which he assigned brood numbers XVIII through XXX (1893 through 1905).[21]

Many of these hypothetical 30 broods, however, have not been observed. Furthermore, two of the brood numbers assigned by Marlatt (Broods XI and XXI) existed at one time, but have become extinct. The Marlatt numbering scheme has been retained for convenience (and because it clearly separates 13- and 17-year life cycles), although today only 15 broods are known to survive.[22]

These seven species are also sometimes grouped differently into three subgroups, the so-called "Decim species group," "Cassini species group," and "Decula species group," reflecting strong similarities of each 17-year species with one or more species with a 13-year cycle.[25]

Generally, the 17-year cicadas are distributed more in the northern states of the eastern United States, while the 13-year cicadas occur in the southern states, but some may overlap slightly. For example, Broods IV (17 year cycle) and XIX (13 year cycle) overlap in western Missouri and eastern Oklahoma.[26][27] Their emergences should again coincide in 2219, 2440, 2661, etc., as they did in 1998[28] (although distributions change slightly from generation to generation and older distribution maps can be unreliable.[27]). An effort is currently underway to generate new distribution maps of all periodical cicada broods. This effort makes use of crowdsourced records and records collected by entomologists.[29]

Not only are the periodical cicada life cycles curious for their prime numbers—13 or 17—their evolution is intricately tied to one- and four-year changes in their life cycle.[16][18] One-year changes are less common than four-year changes and are probably tied to variation in local climatic conditions. Four-year early and late emergences are common and involve a much larger proportion of the population than one-year changes.

Recent research suggests that in extant periodical cicadas the 13- and 17-year life cycle evolved at least eight different times in the last 4 million years and that different species with identical life cycles developed their overlapping geographic distribution by synchronization of life cycle to existing dominant populations.[30] The same study estimates that the Decim species group split from the common ancestor of the Decula plus Cassini species groups around 4 million years ago (Mya). At around 2.5 Mya the Cassini and Decula groups split from each other.

The Sota et al. (2013) paper also calculates that the first separation of extant 13-year cicadas from 17-year cicadas took place in the Decim group approximately 530,000 years ago (ya) when the southern M. tredecim split from the northern M. septendecim. The second noteworthy event took place about 320,000 ya with the split of the western Cassini group from its conspecifics to the east. The Decim and the Decula clades experienced similar western splits but these are estimated to have taken place 270,000 and 230,000 ya, respectively. The 13- and 17-year splits in Cassini and Decula took place after these events.

The 17-year cicadas largely occupy formerly glaciated territory and as a result their phylogeographic relationships reflect the effects of repeated contraction into glacial refugia (small islands of suitable habitat) and subsequent re-expansion during multiple interglacial periods. In each species group, Decim, Cassini, and Decula the signature of the glacial periods is manifested today in three phylogeographic genetic subdivisions: one subgroup east of the Appalachians, one midwestern, and one on the far western edge of their range.

The Sota et al. data suggest that the founders of the southern 13-year cicada populations we see today originated from the Decim group. These were later joined by Cassini originating from the western Cassini clade and Decula originating from eastern, middle, and western Decula clades. As Cassini and Decula invaded the south they became synchronized with the resident M. tredecim. Today these Cassini and Decula are known as M. tredecassini and M. tredecula. More data is needed to lend support to this hypothesis and others hypotheses related to more recent 13-17-year splits involving M. neotredecim and M. tredecim.

Magicicada is edible when cooked. They have historically been eaten by Native Americans, who would roast them in hot ovens, stirring them until they were well browned.[31]

Noted entomologistCharles Lester Marlatt wrote in 1907 that, "The use of the newly emerged and succulent cicadas as an article of human diet has merely a theoretical interest, because, if for no other reason, they occur too rarely to have any real value. There is also the much stronger objection in the instinctive repugnance which all insects seem to inspire as an article of food to most civilized nations. Theoretically, the Cicada, collected at the proper time and suitably dressed and served, should be a rather attractive food. The larvae have lived solely on vegetable matter of the cleanest and most whole-some sort, and supposedly, therefore, would be much more palatable and suitable for food than the oyster, with its scavenger habit of living in the muddy ooze of river bottoms, or many other animals which are highly prized and which have not half so clean a record as the periodical Cicada."[32]

Cicadas are used in alternative medicine and traditional Chinese medicine (TCM). Though the insect itself is not of much importance in traditional Chinese medicine the molted skins of the Cicadas are of greater value, and have been used to treat rheumatoid arthritis as well as other diseases. Traditionally, the skin is mixed with herbs and boiled into a tea which the patient consumes or applies to affected area.[33][34]